Add flour and milk alternately and continue to whisk in low speed until well blended.

take 1/3 of batter and mix in mocha paste. Set aside.

Add the vanilla into the balance of white batter, stir well and set aside.

Fill the cups with white batter then mocha batter alternately until it came almost up to the rim.

Steam the cakes in steamer for 10 minutes. Do not open the steamer's lid before the cakes are done. Give extra space between the molds when steaming the cakes as these cakes need a lot of steams to perfectly rise and crack in the middle.

After 15 minutes, take the cakes out from the steamer. Let them cool down for a while before releasing them from the molds.